Having been the victim of a bike theft last year, I can appreciate the advice to see a receipt or to check with the police to make sure it's not stolen (and, while I couldn't find my receipt for my 10-year old bike that was stolen, my LBS was able to quickly get me a copy of my receipt, with serial number, for the police report). And, as someone who has bought and sold camera equipment online, I am also of the opinion that asking a lot of questions to determine if the seller is the original owner of the item and how it's been stored/handled/used is a key. While I've bought expensive camera equipment online, I will drop out of an ebay listing without hesitation if the seller doesn't have the time to respond fully to specific questions (and respond with enough info that satisfies me that he/she knows enough about what they are selling to be the real owner and not some middleman seller). What I'd be less confident of is my ability to ensure the frame/forks/components on a high end bike don't have any damage or unusual wear on them. It sounds like you may have lucked into a low mileage user who just changed her mind after purchase (or after a spouse/boyfriend/significant other bought it for her, thinking she would use it), but can you have your LBS check it out/tune it up before you buy to give you the peace of mind it's not damaged?

And, if it is gone before you buy, I saw your comment you were also considering a Specialized Amira. As a happy Amira owner, I'd say you couldn't go wrong there either (and the '12 one in my LBS is on sale now).